Solid State NMR Probe Design

The majority of solids NMR experiments today are driven by biological applications in rigid macromolecules, where even the most advanced liquids methods are limited by the spectral resolution they can achieve. Obtaining detailed structural information from NMR of solid samples is often more technically demanding than for liquids because of the absence of molecular tumbling in solids to average linebroadening interactions; hence, much greater probe bandwidths are required. A wide array of line narrowing techniques have been developed over the past twenty years for solid samples that greatly extend the earlier capabilities. The original solids probe, the wideline probe, usually has spectral excitation bandwidth greater than 80 kHz (i.e., able to generate 90 pulses less than 3 μs). To do this, it normally must withstand peak rf voltages in excess of 2 kV – possibly up to 5 kV. Often wideline probes are double-tuned (DT) to permit cross polarization (CP) and high-power decoupling for increased sensitivity and resolution. Multiplepulse line-narrowing techniques may be used to remove dipolar broadening, especially if π/2 pulse lengths below 2 μs can be generated. At very high fields, this is more readily achieved in singleresonance probes, where coils of very low inductance can be used effectively. The Cross-Polarization/Magic Angle Spinning (CPMAS) probe adds high-speed sample spinning at the Magic Angle to the other capabilities of the DT wideline probe for effective averaging of susceptibility broadening, chemical shift anisotropy, and often dipolar broadening, thereby greatly improving resolution for most spin-1/2 nuclei. The 3D spectral capabilities of a triple resonance probe, especially H/C/N, are extremely useful in solving structures of larger molecules. Hence, the triple-resonance MAS probe is quickly becoming the most commonly used solids probe in macromolecule applications. In many cases, spinning rates above 20 kHz, decoupling field strengths above 100 kHz with low rf heating of the sample, very high sensitivity on all three channels, high spectral resolution, and wide variable temperature (VT) range with stable control are important. In some cases, it is also desirable to be able to apply a BB0 gradient at the magic angle and have automatic sample exchange capability. The combination of these features makes such a probe a technically challenging instrument. 7